The Vikings have completed five days of practice and are still rebuilding. But wait, there's more. Here are a trio of other things you should know in this edition of Three-and-Out:

1. Erin Henderson is hoping for a breakthrough season followed by a big payday.

Put it in ink: Henderson will be the opening day starter at weak-side linebacker. And that alone has the 26-year-old feeling assured and confident his ascension will produce a steep curve this season.

Henderson has no trouble admitting his first go-round as a starter in 2011 came with growing pains. He says he too often was enticed to try to make plays that weren't his responsibility, a temptation that grew as the Vikings' struggles mounted.

"I kind of had that college mentality still," Henderson said. "It was, 'Make every tackle and try to be in on every play.' But at this level, you can't do that."

Henderson's biggest wake-up call came late in the season against the Broncos. Straying from his gap to chase Willis McGahee, Henderson left the defense vulnerable and infuriated defensive coordinator Fred Pagac.

"Pug told me, 'If I see that again, you're coming on the sideline next to me.' It was straight like that," Henderson said. "From what I remember, it was nothing too bad -- a 12- or 13-yard run. But he was tired of seeing that. And I understand why. He told me if I get out of the B gap again, he was going to put me on the bench and somebody else was going in there."

Henderson has come to camp with greater discipline plus a hunger to prove his worth. Remember the anxiety he had in March when he sat available on the free-agent market, unable to land the marquee deal he felt he deserved?

The Vikings eventually signed him to a one-year deal, worth around $2 million, setting the stage for a prove-it season.

In late February, Browns linebacker D'Qwell Jackson -- a friend of Henderson's and a former teammate at Maryland -- had received a lucrative contract extension reportedly worth more than $42 million over five years. Jackson's "Your time will come" pep talk encouraged Henderson.

Said the Vikings linebacker: "D'Qwell told me, 'It's going to come when it's supposed to come. You have to keep grinding until it does.' ... Nothing I can do about it right now. I need to go out there and take care of my business and handle my end and hopefully it will play out different next time."

2. John Carlson's absence will be significant in the next few weeks.

Carlson is out until further notice because of a knee injury, a setback he suffered late in Tuesday's practice when, while he was blocking, someone rolled into his right knee. An MRI exam revealed a Grade 2 sprain of the medial collateral ligament, which likely will put Carlson on the shelf for at least two weeks.

So what's the big deal? Well, the Vikings made Carlson one of their top targets in free agency and ended up overwhelming the tight end with a deal worth $25 million over five seasons. That kind of investment brings great expectations. Plus offensive coordinator Bill Musgrave has made it clear he plans to run a lot of two-tight-end sets, envisioning Carlson and Kyle Rudolph as major catalysts to the growth of quarterback Christian Ponder.

None of that is off the table, obviously. If Carlson heals quickly and can be back in action by the third preseason game -- Aug. 24 against San Diego -- the Vikings will be ecstatic. But let's not forget, Carlson has had a handful of significant injury setbacks in recent years. He missed all of 2011 after shoulder surgery. His 2010 season (31 catches, 318 yards) ended in the playoffs with a nasty concussion in Chicago.

As for what will be missed with Carlson on the side for the rest of the team's time in Mankato? Coach Leslie Frazier said the tight end's veteran presence will be lessened. In addition, Frazier said, "The fact that it's a new offense for him -- the terminology, being able to not just hear it in meetings but to and actually practice his movements, you miss that part of it."

3. Greg Childs still has a lot to prove.

When the Vikings used a fourth-round pick to nab Childs in April, they were intrigued by his potential. The 6-3, 217-pound playmaker out of Arkansas sure looks the part, possessing a combination of size, speed and ball skills that has Frazier and General Manager Rick Spielman believing he could emerge as the steal of the 2012 draft.

But the electric version of Greg Childs hasn't been seen very much in the past two years. Childs' junior season in college was cut short when he tore the patella tendon in his right knee. That injury lingered into 2011, with Childs' recovery so slow that he averaged only two catches and 22 yards per game as a senior.

Then he came to the Vikings and was limited during rookie minicamp and into organized team activities because of a calf strain. And now, in the first week of training camp, Childs has had minor difficulty holding on to passes he should catch.

The Vikings' 2012 revival plans will hinge on patience. And that applies to monitoring Childs' growth. Yet Frazier has been straightforward with what he hopes to see.

"You want him to be able to show that he can stay healthy," Frazier said. "That's a big deal, being available."

Ideally, Childs will prove his health is not a worry and that his potential can be realized.

"His size stands out," Frazier said. "To have a big receiver who can run as well as he can and having the catching radius that he has gives you a belief that even when people have decent coverage, because of his size and ability to catch balls in tough spots, he has big-play potential."